Its historical, cultural. I have several childhood friends, Irish or Italian background, whose fathers or grandfathers were blue-collar workers, strong "union" guys - when unions were not government-run, and when they were not Marxist.
So they would feel it as a betrayal of themselves to be anything but Democrats.
But having said that, they also aren't knowledgeable about either present Democrat positions, nor are they studious and practicing "Catholics."
At my traditional latin Church, the priest recently gave a homily about the FBI and coming leftist persecution of Catholics and Christians of all kinds. Quiet different, very conservative, but there is room for diversity of opinion within the Church.
The country-club Brahmins who ran the Republican party in the past wanted nothing to do with Catholics so they became Democrats. Same thing with the Jews.